PCT Publication No. WO97/19819 (Horsell) describes a method of preparing a substrate for a planographic printing plate which comprises contacting a support with a liquid comprising a sodium silicate solution in which alumina and titania are dispersed thereby to produce a hydrophilic layer over the support. In each of the examples, the ratio of the wt % of alumina to titania is 1:1. However, investigations have shown that there are significant problems with blanket piling and undesirable ink-sensitive spots associated with the use of the formulations described.
Paper used on a sheet fed offset printing press is normally smaller than the plate used to print. "Blanket piling" is the build-up of ink on the blanket within the area contacted by the plate but not by the paper. It can lead to problems over longer press runs by transferring ink to the edge of the paper being printed. In addition, the press needs to be stopped more frequently for blanket cleaning where piling is a problem. On conventional substrates, the cause of piling is usually due to poor ink/water balance, non-waterproof ink or the surface chemistry and morphology of the substrate.
Ink-sensitive spots have been observed on printing plates prepared as described in WO97/19819 after the plates have been cleaned either during or after a print run and then, subsequently, re-run on press--the spots appear once the press is restarted. Although the spots are removable by cleaning the plates for a second time, the effect is undesirable since, if the plates are not properly cleaned (as could easily happen in a commercial printing environment), the spots would result in ink being transferred to printed sheets which would, therefore, be rendered useless.
Other problems that have been identified in relation to the formulations described in WO97/19819 are associated with its application to a support. In this regard, firstly, the particulates in the formulations have a tendency to settle out of the formulations too quickly; secondly, the viscosity of the formulations is relatively low which tends to increase the formulations' instability; and, thirdly, the formulations are relatively abrasive which leads to significant wear on apparatus used to coat: them.
It is a primary object of the present invention to address the piling problem described above. It is another object to address the ink spot problem described above. It is a further object to address other problems associated with the formulations described in WO97/19819.